Non-traumatic progressive paralysis of the posterior interosseous nerve.

Isolated lesions of the posterior interosseous nerve of the forearm are uncommon. Most follow injury to the elbow region The clinical features of a long-standing lesion of the nerve are wasting of the posterior forearm muscles (excepting brachio-radialis and extensor carpi radialis longus), diminution or loss of active extension of the fingers at the metacarpo-phalangeal joints, and weakness or paresis of the abductor pollicis longus muscle. There is no wrist drop as the nerve supply to the extensor carpi radialis longus is intact, but on extension the wrist tends to drift into radial deviation because of the paralysis of extensor carpi ulnaris muscle. A number of cases have now been reported of an almost painless progressive paralysis of this nerve, usually unexplained. ANATOMY The radial nerve terminates a few centimetres above the lateral condyle of the humerus, dividing into the posterior interosseous nerve and the lateral cutaneous nerve of the forearm. Before this division, branches of supply are given to the brachialis, brachio-radialis and extensor carpi radialis longus muscles. The posterior interosseous nerve descends in the cleft between the brachialis and brachio-radialis, passes under the extensor carpi radialis longus and brevis, supplying the latter muscle, and then pierces the supinator. It curves around the radius in the substance of the supinator and usually, at the lower border, divides into two branches, one of which supplies the medial extensor muscles-extensor carpi ulnaris, extensor digitorum and extensor digiti minimi-and the other supplies the abductor pollicis longus and brevis and the extensor indicis muscles. Occasionally some of the fibres of the latter branch pass entirely superficial to the supinator or become superficial earlier in their course (Luschka and Krause 1927). The nerve is well overlapped by muscle throughout its course, and over the extensor muscles there is a tough unyielding aponeurosis. PREVIOUS REPORTS I have been able to find twenty reported cases of paralysis of the posterior interosseous nerve apparently unrelated to injury. In only seven of these has exploration been carried out. Agnew (1863) explored the forearm of a woman who had both flexor and extensor weakness. An exquisitely tender tumour " about the size of a hickory nut " was palpable anteriorly close to the biceps tendon. Exploration of this swelling showed it to be a solid lesion containing fibrous and connective tissue, lying between the tubercle of the radius and the biceps tendon. It was apparently compressing both the posterior …